1. Field of the Invention
This invention relates to an improved process for coating the surface of a transparent refractory material such as glass with a adherent film of tin oxide by projecting a pyrolytically decomposable tin compound onto the surface of a heated substrate.
2. Description of the Prior Art
Processes for the pyrolytic deposition of metallic oxide films on surfaces of transparent refractory materials, such as glass sheets, are well known. In general, the substrate on which the deposition is accomplished is heated to a high temperature but one below its softening point. A tin compound that decomposes by pyrolysis upon contact with the heated substrate is deposited on the substrate's surface. As a result of the ensuing chemical reaction, a heat-reflective, transparent, adherent film of tin oxide is formed on the surface of the substrate.
French Pat. No. 2,277,049 discloses a process in which an organometallic compound in granular form is suspended in an adhydrous gas current which is projected against the heated surface of the substrate to be coated. These prior art processes, however, all possess significant drawbacks with respect to continuity of the final coating or the difficulty involved in applying the coating.
U.S. Pat. No. 4,172,159 discloses a process for coating the surfaces of a transparent refractory material that allegedly avoids the problems existing in the more conventional prior art processes. In this process, the anhydrous carrier gas contains, in addition to finely divided particles of an organotin compound, an anhydrous gaseous fluoride compound which is introduced into the carrier gas at a point upstream from the nozzle that projects the gas stream onto the surface of the substrate.
It has been discovered, however, that this process does not insure a constant ratio between the amounts of tin and fluorine deposited onto the surface of the substrate. A constant ratio between the two constituents is required in order to produce uniform layers of coating. This is important in all commercial applications, particularly in manufacturing transparent glass used in constructing windows. Moreover, because of the manner in which the fluoride and tin compounds are mixed, the process involves the use of costly regulating devices which still do not insure that a uniform, homogeneous layer will be deposited.
The improved process of the present invention completely eliminates the necessity for employing costly regulating devices, allows the tin/fluoride ratio of the coating to be precisely regulated, and produces homogeneous layers which are superior to those produced by any of the prior art processes.